This invention relates to a variable impedance circuit of the type having an electrically variable impedance element which utilizes the forward-biased diode characteristic of a transistor base-emitter junction.
A known type of electrically variable impedance control circuit is disclosed in my prior U.S. Pat. No. 3,761,741, which is hereby incorporated by reference in its entirety. As compared to prior art configurations, the invention of my prior patent is able to provide a wide range of impedance values while maintaining a high degree of linearity. Whereas prior art circuits, such as that shown in FIG. 1 of my prior patent, may have a nonlinearity factor as high as 30% due to variations of impedance induced by the signal source, a circuit in accordance with FIG. 4 of my prior patent is capable of operating over a wide impedance range with nonlinearities in the order of only 1.5%.
However, in order to achieve a larger range of electrical impedance values and a larger output swing with my prior art configuration, it is necessary to connect a plurality of diode-connected transistors in series, as shown in FIG. 5 of my earlier patent. While such a circuit is capable of providing a wide range of impedance values as well as a substantial improvement in linearity as compared to prior art devices, this circuit still suffers from a number of significant drawbacks.
Since the circuit of FIG. 5 of my prior patent requires that a series string of ten base-emitter junctions be biased into the active region, and since these base-emitter junctions are connected in series with two further transistors, it is apparent that this prior art circuit requires a power supply voltage of at least about 8 volts for proper operation. In fact, the circuit of FIG. 5 of my prior patent has been used as the variable impedance element of a commercially-produced Dolby processor integrated circuit, and this circuit requires a power supply voltage of at least 8 volts for proper operation. Furthermore, this prior art circuit requires the use of sixteen diode-connected transistors, and even more such transistors must be provided if higher voltage swings are needed. Clearly, such a configuration is both complex and wasteful of chip area.
With the increasing popularity of portable, battery-operated equipment, it has become desirable to achieve the superior performance of my prior variable impedance invention in a circuit capable of operating at lower voltages. In particular, it would be desirable to power such a circuit from a nominal 6 volt battery source, with the circuit capable of functioning down to a minimum of about 4 volts.